The pistillate strawberry cultivars ‘Freya’ and ‘Tardive de Leopold’ were used for experiments designed to study the effects on fruit development of 4-(indol-3-yl)-butyric acid (IBA) and 2-naphthoxyacetic acid (2NoA), each applied alone or in combination with gibberellic acid (GA) GA was also applied alone. Methods of application tested included the treatment of individual flowers with solutions of the growth promoters in lanolin emulsions or in agar gels, and the use of aqueous solutions as overall sprays.
Applying growth substances in lanolin or agar carriers was highly effective as a means of inducing parthenocarpic development. In ‘Freya’, 2NoA at 1,000 p.p.m. in lanolin induced the production of fruits with approximately the same mean weight as those from pollinated control treatments. IBA was less effective, and at concentrations of up to 1,000 p.p.m. failed to induce appreciable development in ‘Tardive de Leopold’; in ‘Freya’ some development occurred at 1,000 p.p.m. but none at 250 or 500 p.p.m. GA applied alone at 1,000 or 2,000 p.p.m. promoted some development in ‘Freya’ only, but most of this was due to growth in the ‘neck’ region below the level of carpel insertion. Combined applications of auxin and gibberellin showed strong synergistic effects on fruit development and resulted in fruit equal or nearly equal in size to that of the control treatments throughout the range of concentrations tested. The period of development was also reduced by treatment with GA.
Attempts to reproduce these results by similar treatments applied as overall aqueous sprays were largely unsuccessful. Increases in fruit size occurred in a few experiments but were neither consistently obtained nor sufficiently great to be useful.